This invention relates to electronic control circuits and more particularly to a tone control circuit for a high fidelity sound reproduction receiver or the like.
The best known tone control circuit of the prior art is the Baxandall circuit which is the basis for many tone controls used in high fidelity sound reproducing equipment. Fundamentally, the Baxandall circuit is used for varying the treble and bass tones by varying the frequency response of an alternating current signal path to an amplifier circuit. The treble control causes the high frequency response to be either increased or decreased with respect to the middle and low frequencies. The base control provides the same type of control to the low frequencies with respect to the middle and high frequencies. The basic circuit comprises two linear potentiometers, three resistors, and three capacitors connected between an input terminal, and, for example, the base to emitter electrodes of a transistor amplifier. One such circuit is required for each channel of a high fidelity reproduction system. Thus, for a two channel stereo system, to control the treble and bass tones would require at least four potentiometers, six resistors, and six capacitors. It is necessary that each pair of potentiometers (two each for treble tones and two each for bass tones) be ganged together. As the number of channels increases the number of potentiometers to be ganged together increases as well as the total number of the components such as the capacitors and resistors comprising the circuit.
One disadvantage of the above circuit is that each ganged potentiometer must be matched with its pair or pairs otherwise the fidelity of the stereo system may suffer. Matching potentiometers becomes expensive and even increases, expectedly, as the number of channels constituting the stereo system increase. Furthermore, the excessive number of capacitors required also increases the expense of stereo systems utilizing the circuit of the prior art.
Another disadvantage of the prior art circuit is that tone control is accomplished in the alternating current signal path, i.e., the potentiometers attenuate the actual audio signal being reproduced in the high fidelity system. This presents a major problem in the design and layout of the high fidelity system. One of the worst problems facing a high fidelity system manufacturer is the running of leads from, for example, a printed circuit board to each potentiometer of the tone control circuit. As the number of ganged potentiometers in the tone control circuit increases as the number of channels increases, so does the number of leads required to be carried to the potentiometers. Because all of the leads to the potentiometers are carrying the audio signal, they are susceptable to hum pick-up from the power supply and other sources. The leads cannot be decoupled because they are carrying the audio signals and therefore the whole layout of the stereo system has to be designed to avoid picking up spurious signals on the leads. More often than not, the leads used in such prior art stereo systems are screen leads. Thus, the problems of layout plus the number of leads and the fact that the leads must be screened also increases the cost of the prior art circuit.
Moreover, because the prior art system requires individual potentiometers to control the treble and bass tones for each channel of the stereo system, it is not readily adaptable to be controlled by a remote control unit. There is a strong tendency for manufacturers of sound reproduction systems to provide, to the consumer, a means for remotely controlling the adjustment of the volume and well as the tone controls of the system. Thus, the consumer would have a hand held remote control unit similar to those provided for televisions, for adjusting the sound quality and volume.
Thus, a need exists for an improved tone control circuit for high fidelity reproduction system which overcomes the above problems of the prior art. Moreover, the tone control circuit of the invention is suitable to be remotely adjusted.